In a light emitting diode using nitride semiconductors, a p-type semiconductor layer has lower electrical conductivity than an n-type semiconductor layer. As a result, electric current is not effectively spread in the p-type semiconductor layer in the horizontal direction, thereby causing current crowding in a specific region of the semiconductor layer. If current crowding occurs in the semiconductor layer, the light emitting diode can become susceptible to electrostatic discharge and can suffer from current leakage and efficiency drooping.
In general, an indium tin oxide (ITO) layer is used to provide current spreading in a p-type semiconductor layer. Since the ITO layer is optically transmissive and has electrical conductivity, the ITO layer can achieve current spreading over a large area of the p-type semiconductor layer. However, the ITO layer has a limit in thickness increase due to light absorptivity thereof. Accordingly, there is a limit in current spreading using the ITO layer.
To assist in current spreading using a transparent electrode such as an ITO layer, a current blocking layer (CBL) can be formed under the transparent electrode. Generally, when a process of etching an active layer and a p-type semiconductor layer to expose an n-type semiconductor layer (hereinafter, mesa etching process) is performed prior to a CBL forming process, a location at which the CBL will be formed is determined with reference to the exposed n-type semiconductor layer. As such, since the mesa etching process is performed prior to the CBL forming process, the mesa etching process and a process of etching the transparent electrode, such as an ITO layer, are separately performed. These processes require separate masks for patterning, thereby providing process inconvenience and increasing manufacturing costs.
In recent years, there is an attempt to use a ZnO transparent electrode layer instead of an ITO layer. Since the ZnO transparent electrode layer has lower light absorptivity than the ITO layer, the ZnO transparent electrode layer can be formed to a greater thickness than the ITO layer, thereby providing better current spreading efficiency than the ITO layer.
However, a transparent electrode using typical ZnO exhibits poor electrical characteristics and causes increase in forward voltage Vf when applied to a light emitting diode. Moreover, typical ZnO has low production yield and thus is not suitable for mass production of light emitting diodes.